What is a paint slot (compared to an image with a UV mapping)?
A texture painting slot is how you can control which is the active target for paint without having the node editor open to hunt and find that corresponding image node to click it to make it active, nor having to find the corresponding texture in the material texture properties.

The origin of the texture slot panel was actually a GSOC adaptation of the Texture Paint Layer add-on that offered similar functionality in the N Panel, and it also gave the ability to not need the properties editor to be open to determine which image texture was to be the target.

Masking is much more than the Stencil type that is normally used between image textures, and the actual mask tool is very powerful and I have explored it a few times on my youtube channel. You can utilize a different UV projection for masking, or you can use the same as the existing UV mapping for the target image.